Thermal ink jet printers are well known in the art and are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices, (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988), and in U.S. Pat. Nos. 4,490,728 and 4,313,684. The thermal ink jet printhead has an array of precisely formed nozzles, each having a chamber that receives ink from an ink reservoir. Each chamber has a thin-film resistor, known as a thermal ink jet resistor, located opposite the nozzle so ink can collect between the nozzle and the thermal ink jet resistor. When electric printing pulses heat the thermal ink jet resistor, a small portion of the ink abutting the thermal ink jet resistor vaporizes and ejects a drop of ink from the printhead. The ejected drops collect on a print medium to form printed characters and images.
Uncontrolled printhead temperature fluctuations have prevented the realization of the full potential of thermal ink jet printheads. These fluctuations produce variations in the size of the ejected drops and this results in degraded print quality. The size of ejected drops varies with printhead temperature because two properties that control the size of the drops (i.e., the viscosity of the ink and the amount of ink vaporized by an addressed resistor) vary with printhead temperature. Printhead temperature fluctuations commonly occur during printer startup, during changes in ambient temperature, and when the printer output varies. For example, temperature fluctuations occur when the printer output changes from normal print to "black-out" print (i.e., where the printer covers the page with ink dots).
When printing text in black and white, the darkness of the print varies with printhead temperature because the darkness depends on the size of the ejected drops. When printing gray-scale images, the shade of gray printed depends on the number of dots in a super pixel and the size of those dots. A super pixel has the ability to hold anywhere from zero dots to a maximum number of dots such as sixteen. One dot in the super pixel produces the lightest shade of gray and the darkest shade of gray occurs when dots cover the super pixel. (For more information on super pixels in thermal ink jet printers see page 350-352 of Output Hardcopy Devices, ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). Ideally, the super pixel becomes covered with ink only when it contains the maximum number of dots. When the uncontrolled printhead temperature gets too high, it produces excessively large dots which have the effect of compressing the range of gray-scale tones. The large dots compress the darker end of the gray-scale range by using fewer than the maximum number of drops to cover the super pixel. Once ink has covered the super pixel, additional drops do not make its tone much darker. The large dots eliminate the lightest tones in the gray-scale range by covering a larger portion of the super pixel and thereby eliminate those gray-scale tones that result from less coverage. Additionally, large dots produced by uncontrolled temperatures result in a noncontinuous gray-scale range because the tone of a blank page, which has the maximum light reflection, is much lighter than the lightest shade of gray. Therefore, the temperature of the printhead must be controlled to obtain a large and continuous range of gray-scale tones.
When printing color images, the printed color varies with printhead temperature because the printed color depends on the sizes of all the primary color drops that create the printed color. If the printhead temperature varies from one primary color nozzle to another, the size of drops ejected from one primary color nozzle will differ from the size of drops ejected from another primary color nozzle. So, the resulting printed color will differ from the intended color. If all the nozzles of the printhead have the same temperature but the printhead temperature increases or decreases as it prints the page, the colors at the top of the page will differ from the colors at the bottom of the page. To print text, graphics, or images of the highest quality, the printhead temperature must remain constant.
Thermal printers are well known in the art. In thermal printers, the heat travels directly to the ribbon or the thermal paper instead of being carried away by an ejected drop. The printheads have an array of heating elements that either heat thermal paper to produce a dot on the thermal paper or heat a ribbon (which can have bands of primary color inks as well as black ink) to transfer a dot to the page. In either case, fluctuations in the printhead temperature produce fluctuations in the size of the printed dot that affects the darkness of the print when printing in black and white, the gray-tone when printing in gray scale, and the resulting printed color when printing in color. The discussion below relating to thermal ink jet printers applies to thermal printers.